The present invention relates generally to a technique for making a curve relating an absorbance to a concentration of substances to be detected photometrically and more particularly to a method and an apparatus for correcting a non-linear curve which is obtained by using at least three standard samples having known concentration.
As a method of quantitatively analyzing a given substance contained in a sample, there have been developed colorimetric analysis, atomic absorption analysis, fluorometric analysis, luminous spectrum analysis, etc. In these analyses, prior to measurement, a curve relating an absorbance to a concentration of substance is derived by using a plurality of standard samples having known or predetermined concentrations of the relevant substance, and during the actual measurement, a concentration of an unknown sample is derived from a measured absorbance value with reference to the previously derived curve. In usual analysis, the above mentioned curve is obtained by using two standard samples having known different concentrations. That is to say, a linear curve is drawn by connecting two measured absorbance values. However, such a linear curve could not be applied to an analysis which requires a high precision. In this connection, it should be noted that in modern chemical analyzers, the measurement should be effected precisely, because an amount of an available sample is made extremely small. Therefore, in the analysis requiring the high precision, it is preferable to form a non-linear curve relating to an absorbance to a concentration by using at least three standard samples having known different concentrations. That is to say, in a colorimetric analysis for use in a biochemical analysis, four standard samples having known concentrations x.sub.1, x.sub.2, x.sub.3 and x.sub.4 are used to derive four absorbance values y.sub.1, y.sub.2, y.sub.3 and y.sub.4, respectively and a non-linear curve A representing a mutual relationship between the absorbance and concentration is formed as illustrated in FIG. 1. Hereinafter, this curve is referred to as an absorbance-concentration curve. In case of making the curve by using more than two standard samples, the curve usually becomes a non-linear curve.
In the colorimetric measurement, it has been often encountered that the absorbance value for the same standard sample fluctuates during the measurement due to various causes such as modification of reagent, variation of ambient temperature, secular variation of sample and reagent and fluctuation of light intensity of a light source. Therefore, the absorbance-concentration curve A changes into a curve B shown in FIG. 1. That is to say, even in such a case if the old curve A is used, a concentration of a sample might be erroneously determined to be a smaller value than an actual value. Therefore, the absorbance-concentration curve A must be corrected during the measurement. Heretofore, this correction for the curve is effected by deriving new absorbance values y.sub.1 ', y.sub.2 ', y.sub.3 ' and y.sub.4 ' for all the four standard samples having the known concentrations x.sub.1, x.sub.2, x.sub.3 and x.sub.4 and the new curve B is formed by means of the newly measured absorbance values. However, such a known correcting method has several drawbacks that it is quite cumbersome to measure absorbance values for all standard samples every time the absorbance-concentration curve has to be corrected and thus a treating ability of a whole analyzer becomes lowered, and that error might be introduced in measuring the absorbances of all the standard samples and thus measuring results might be affected.
In case of correcting the linear absorbance-concentration curve, it has been known to shift the curve vertically by an amount equal to a difference between the old absorbance value and the newly measured absorbance value of one standard sample. However, this known correcting technique could not be applied to the non-linear curve, because if so, the non-linear curve might be shifted from an origin at which both the concentration and absorbance values are zero.